1. Field of the Invention
This invention relates to methods and apparatus for specifically isolating tumor cells from a population of non-tumor cells. In particular, this invention relates to methods and apparatus for specifically labeling and thereafter individually killing tumor cells with a focused high-energy beam such as a laser beam.
2. Description of the Related Art
Hematopoietic stem cell transplantation is a rapidly growing therapy throughout the world. Hematopoietic stem cells are cells that reside in the bone marrow and lead to the production of all of the body's blood cells. In 1995 over twenty thousand hematopoietic stem cell transplants were performed in the United States. In particular, the treatment of breast cancer with autologous hematopoietic stem cell transplantation has become a widely used cancer therapy.
Tumor metastasis is a well-known process by which tumor cells leave their initial location and spread to other parts of the body. Once transported to a new site, the tumor cells begin to grow and populate the new site, thus creating a new tumor. One treatment for patients with metastatic tumors involves harvesting their hematopoietic stem cells and then treating the patient with high doses of radiotherapy or chemotherapy. This treatment is designed to destroy all the patients tumor cells, but has the side effect of also destroying their hematopoietic cells. Thus, once the patient has been treated, the autologous stem cells are returned to their body.
However, if the tumor cells have metastasized away from the tumor's primary site, there is a high probability that some tumor cells will contaminate the harvested hematopoietic cell population. In such a case, the harvested hematopoietic stem cells include contaminating tumor cells. It is important to find a mechanism for killing all of the metastasized tumor cells prior to reintroducing the stem cells to the patient. If any living tumorigenic cells are re-introduced into the patient, they can lead to a relapse.
The problem of removing tumor cells from hematopoietic cells has been reported during traditional bone marrow harvest procedures (Campana, D. et al. Detection of minimal residual disease in acute leukemia: methodologic advances and clinical significance, Blood, Mar. 15, 1995 85(6): 1416–34). Similar problems were also found when others attempted to remove tumor cells with the newer method of leukopheresis of mobilized peripheral blood cells (Brugger, W. et al. Mobilization of tumor cells and hematopoietic progenitor cells into peripheral blood of patients with solid tumors Blood, 83(3): 636–40, 1994).
In each of these procedures, the number of contaminating tumor cells ranged from about 10 to 5000 tumor cells per four million mononuclear harvested cells, depending on the chemo-therapeutic drug regimen used for mobilization. Mononuclear cells were obtained by a discontinuous density gradient centrifugation of the entire hematopoietic cell harvest. The total number of mononuclear cells harvested from a patient is normally on the order of 10 billion cells. Thus, the total tumor burden in a harvest varies from a lower boundary of about 25 thousand cells to a higher boundary of about 12 million cells.
These contaminating tumor cells have been shown by genetic marking to contribute to tumor relapse (Rill, E R et al., Direct Demonstration That Autologous Bone Marrow Transplantation for Solid Tumors Can Return a Multiplicity of Tumorigenic Cells Blood, 84(2): 380–383, 1994). Thus, a great need exists for efficient methods for removing all of the tumor cells from a hematopoietic cell transplant (Gulati, S C et al. Rationale for purging in autologous stem cell transplantation. Journal of Hematotherapy, 2(4):467–71, 1993). A rapid and reliable method for removing all of the contaminating tumor cells would improve the efficacy of hematopoietic stem cell transplantation for a growing number of patients.
Others have attempted to remove contaminating tumor cells from hematopoietic stem cell harvests, but have met with limited success. Several methods of purging the tumor cell populations away from the harvested stem cells have been proposed and tested (A. Gee, Editor Bone Marrow Processing and Purging, Part 5, CRC Press, Boca Raton, Fla., 1991). Thus, the idea underlying all of these purging methods is to separate or destroy the malignant cells while preserving the hematopoietic stem cells that are needed for hematopoietic reconstitution in the transplantation patient.
Some companies and physicians have attempted to purge malignant cells away from populations of non-tumor cells using an immunoaffinity bead-based selection. In this procedure, the total cell population is contacted by immunoaffinity beads. For example, to isolate tumor cells from hematopoietic cells, a first (positive) CD34 selection isolates hematopoietic cells from tumor cells. Binding hematopoietic-cell-specific anti-CD34 antibodies to the immunoaffinity beads allows the physician to specifically remove these cells from populations of non-hematopoietic cells. In some instances, a negative immunoaffinity bead-based selection is also run on tumor or epithelial cell markers by conjugating tumor-specific antibodies to the beads.
Another method that has been tried for removing tumor cells from populations of non-tumor cells involved immunoconjugating a toxic agent to an antibody having specificity for only the tumor cells. In this system, antibodies were bound to chemotoxic agents, toxins, or radionucleides and then contacted with the harvested cell population. Unfortunately, not all of the tumor cells were killed by this treatment.
Other systems for isolating tumor cells from non-tumor cell populations have used the non-specific binding characteristics of hematopoietic cells as the basis for separation. For example, Dooley et al. used these adhesive characteristics to isolate hematopoietic cells with deep bed filtration (Dooley D C et al., A novel inexpensive technique for the removal of breast cancer cells from mobilized peripheral blood stem cell products, Blood, 88(10)suppl 1: 252a, 1916). However, some of the tumor cells were found to isolate with the hematopoietic cells, thus opening the door for a potential relapse by the patient.
In addition, cytotoxic agents, such as 4-Hydroxy-peroxy-cyclo-phosphamide (4HC), have been used to selectively kill tumor cells without damaging the hematopoictic stem cells. Unfortunately, this system also led to lower harvests of hematopoietic cells because the cytotoxic agents weakened or destroyed some of the non-tumor cells.
In other methods, sensitizing agents, such as merocynanine, were mixed with the cell populations which were thereafter photo-irradiated to specifically kill the tumor cells (Lydaki et al. Merocyanine 540 mediated photoirradiation of leukemic cells Journal of Photochemistry and Photobiology 32(1–2):27–32., 1996). Also, Gazitt et al. used fluorescence activated cell sorting (FACS) to sort hematopoietic stem cells from tumor cells (Gazitt et al. Purified CD34+ Lin− Thy+ stem cells do not contain clonal myeloma cells Blood, 86(1):381–389, 1995). As is known, flow cytometry sorts cells one at a time and physically separates one set of labeled cells from another second set of cells. However, it has been shown that individual neurons can be killed after loading them with the adsorbing dyes used in flow cytometry (Miller, J P and Selverston A I., Rapid Killing of Single Neurons by irradiation of Intracellularly injected dye Science, 206:702–704, 1979). Thus, using FACS to separate cell populations is not advantageous because the cell yields can be very low.
In another protocol, Clarke et al. discolsed the use of adenovirus mediated transfer of suicide genes to selectively kill tumor cells (Clarke et al. A recombinant bc1-x s adenovirus selectively induces apoptosis in cancer cells but not in normal bone marrow cells Proc. Nat. Acad. Sci. 92(24):11024–8, 1995).
However, most of the methods listed above are based on administering a whole-population based tumor cell separation or killing strategy. Unfortunately, the whole population tumor purging methods listed above do not kill or remove all of the contaminating tumor cells from the harvested stem cell population. In the best case, the residual tumor cell burden remained at 1 to 10 tumor cells for every 100,000 cells present in the initial harvest (Lazarus et al. Does in vitro bone marrow purging improve the outcome after autologous bone marrow transplantation? Journal of Hematotherapy, 2(4):457–66, 1993).
Therefore, even using the best available techniques, the number of residual tumor cells that are reintroduced into the patient during autologous stem cell transplantation is on the order of 10 to 2000 cells. Given the rapid exponential growth of tumor cells, such residual tumor cells in the transplant can quickly lead to a patient's relapse.
A still further method utilizing laser technology is described in U.S. Pat. No. 4,395,397 to Shapiro. In the Shapiro method, labeled cells are placed in a flow cytometer having a fluorescence detector to identify the labeled tumor cells. A laser beam is used to kill the labeled cells as they pass by the detector. This method suffers from a number of disadvantages. Firstly, once an unwanted cell has passed through the detector/laser region there is no way to check that destruction has been completed successfully. If a tumor cell evades destruction it will inevitably be reintroduced into the patient. Secondly, the focal spot diameter of the laser beam is of necessity greater than the liquid stream cross section. Accordingly, many of the cells in the region of an unwanted cell will be destroyed by the laser beam, including healthy cells.
Another method that utilizes laser technology is described in U.S. Pat. No. 5,035,693 to Kratzer. In this method, labeled cell populations are placed on a moving belt. The labeled cells are thereafter identified by a detector and destroyed by a laser. However, this system has many of the same disadvantages as the Shapiro method. For example, because the cells are moving on a belt past the detector in one direction, the method is not reversible. Thus, if a single tumor cell escapes detection, it will be reintroduced into the patient.
Thus, in spite of extensive efforts and many innovative approaches, there exists a great and growing need for methods and systems for eradicating virtually every tumor cell from a harvested cell population. The system and method described herein fulfills this need.